Voltage-dependent stimulation of Na+/K(+)-pump current by external cations: selectivity of different K+ congeners

Mechanism of potassium ion uptake by the Na(+)/K(+)-ATPase

The Na⁺/K⁺-ATPase restores sodium (Na⁺) and potassium (K⁺) electrochemical gradients dissipated by action potentials and ion-coupled transport processes. As ions are transported, they become transiently trapped between intracellular and extracellular gates. Once the external gate opens, three Na⁺ ions are released, followed by the binding and occlusion of two K⁺ ions. While the mechanisms of Na⁺ release have been well characterized by the study of transient Na⁺ currents, smaller and faster transient currents mediated by external K⁺ have been more difficult to study. Here we show that external K⁺ ions travelling to their binding sites sense only a small fraction of the electric field as they rapidly and simultaneously become occluded. Consistent with these results, molecular dynamics simulations of a pump model show a wide water-filled access channel connecting the binding site to the external solution. These results suggest a mechanism of K⁺ gating different from that of Na⁺ occlusion.

During transport by the Na⁺/K⁺-ATPase, Na⁺ and K⁺ ions become occluded between intra- and extracellular gates. Here Castillo et al. measure transient electrical signals arising from K⁺ occlusion and use molecular simulations to describe a K⁺ gating mechanism fundamentally different to that of Na⁺.

Apoptotic insults impair Na+, K+-ATPase activity as a mechanism of neuronal death mediated by concurrent ATP deficiency and oxidant stress

The Na+, K+-ATPase (Na+, K+-pump) plays critical roles in maintaining ion homeostasis. Blocking the Na+, K+-pump may lead to apoptosis. By contrast, whether an apoptotic insult may affect the Na+, K+-pump activity is largely undefined. In cultured cortical neurons, the Na+, K+-pump activity measured as a membrane current Ipump was time-dependently suppressed by apoptotic insults including serum deprivation, staurosporine, and C2-ceramide, concomitant with depletion of intracellular ATP and production of reactive oxygen species. Signifying a putative relationship among these events, Ipump was highly sensitive to changes in ATP and reactive oxygen species levels. Moreover, the apoptosis-associated Na+, K+-pump failure and serum deprivation-induced neuronal death were antagonized by pyruvate and succinate in ATP- and reactive-oxygen-species-dependent manners. We suggest that failure of the Na+, K+-pump as a result of a combination of energy deficiency and production of reactive oxygen species is a common event in the apoptotic cascade; preserving the pump activity provides a neuroprotective strategy in certain pathological conditions.

Comparison of coupled and uncoupled currents during glutamate uptake by GLT-1 transporters

The transport of glutamate across the plasma membrane is coupled to the movement of cations (Na+, K+, and H+) that are necessary for glutamate uptake and transporter cycling as well as anions that are uncoupled from the flux of glutamate. Although the relationship between these coupled (stoichiometric) and uncoupled (anion) transporter currents is poorly understood, transporter-associated anion currents often are used to monitor transporter activity. To define the kinetic relationship between these two components, we have recorded transporter currents associated with stoichiometric and anion charge movements occurring in response to the rapid application of l-glutamate to outside-out patches from human embryonic kidney cells expressing GLT-1 transporters. Transporter-associated anion currents were approximately twice as slow to rise and decay as stoichiometric transport currents, but the presence of permeant anions did not slow transporter cycling. A kinetic model for GLT-1 was developed to simulate the behavior of both components of the transporter current and to estimate the capture efficiency of GLT-1. In this model the K+ counter-transport step was defined as rate-limiting, consistent with the slowing of transporter cycling after the substitution of internal K+ with Cs+ or Na+. The model predicts that in physiological conditions approximately 35% of GLT-1 transporters function as buffers, releasing glutamate back into the extracellular space after binding.

Electrophysiology of the sodium-potassium-ATPase in cardiac cells

Like several other ion transporters, the Na(+)-K(+) pump of animal cells is electrogenic. The pump generates the pump current I(p). Under physiological conditions, I(p) is an outward current. It can be measured by electrophysiological methods. These methods permit the study of characteristics of the Na(+)-K(+) pump in its physiological environment, i.e., in the cell membrane. The cell membrane, across which a potential gradient exists, separates the cytosol and extracellular medium, which have distinctly different ionic compositions. The introduction of the patch-clamp techniques and the enzymatic isolation of cells have facilitated the investigation of I(p) in single cardiac myocytes. This review summarizes and discusses the results obtained from I(p) measurements in isolated cardiac cells. These results offer new exciting insights into the voltage and ionic dependence of the Na(+)-K(+) pump activity, its effect on membrane potential, and its modulation by hormones, transmitters, and drugs. They are fundamental for our current understanding of Na(+)-K(+) pumping in electrically excitable cells.

Effects of extracellular ions on the reactivation of human spermatozoa preserved in electrolyte-free solution

It has previously been reported that human spermatozoa preserved in an electrolyte-free solution can survive for several weeks at 4 degrees C. However, the motility of spermatozoa after preservation cannot be restored when incubated at 37 degrees C, unless reactivated by extracellular alkalisation. Under weak acidic conditions, the reactivation is induced by > or = 10 mmol l-1 Na+ and inhibited by a Na(+)-H+ antiporter inhibitor. The addition of > or = 0.1 mmol l-1 K+ also induces the reactivation. In the present study, the reactivation was induced by > or = 0.1 mmol l-1 Rb+ or > or = 1 mmol l-1 Cs+ at an acidic pH. The maximum motility rate with K+, Rb+ or Cs+ was obtained at 10-20 mmol l-1 and inhibited by 10(-5)-10(-2) mol l-1 ouabain in a dose-dependent manner, while ouabain had no effect on the Na(+)-induced reactivation. The addition of K+ further increased sperm motility reactivated by Na+, which was also inhibited by ouabain. The addition of Ca2+ did not induce the reactivation or increase sperm motility reactivated by Na+ or K+. It was concluded that activation of the ouabain-sensitive Na(+)-K(+)-ATPase and Na(+)-H+ exchange mechanism has an important role in sperm motility.

Modulation of the gating of CIC-1 by S-(-) 2-(4-chlorophenoxy) propionic acid

1. Using whole-cell patch-clamping and Sf-9 cells expressing the rat skeletal muscle chloride channel, rCIC-1, the cellular mechanism responsible for the myotonic side effects of clofibrate derivatives was examined. 2. RS-(+/-) 2-(4-chlorophenoxy)propionic acid (RS-(+/-) CPP) and its S-(-) enantiomer produced pronounced effects on CIC-1 gating. Both compounds caused the channels to deactivate more rapidly at hyperpolarizing potentials, which showed as a decrease in the time constants of both the fast and slow deactivating components of the whole cell currents. Both compounds also produced a concentration-dependent shift in the voltage dependence of channel apparent open probability to more depolarizing potentials, with an EC50 of 0.79 and 0.21 mM for the racemate and S-(-) enantiomer respectively. R-(+) CPP at similar concentrations had no effect on gating. RS-(+/-) CPP did not block the passage of Cl- through the pore of rCIC-1. 3. CIC-1 is gated by Cl- binding to a site within an access channel and S-(-) CPP alters gating of the channel by decreasing the affinity of this binding site for Cl-. Comparison of the EC50 for RS-(+/-) CPP and S-(-) CPP indicates that R-(+) CPP can compete with the S-(-) enantiomer for the site but that it is without biological activity. 4. RS-(+/-) CPP produced the same effect on rCIC-1 gating when added to the interior of the cell and in the extracellular solution. 5. S-(-) CPP modulates the gating of CIC-1 to decrease the membrane Cl- conductance (GCl), which would account for the myotonic side effects of clofibrate and its derivatives.

A new system for the measurement of electrogenicity produced by ion pumps using a thin polymer film: examination of wild type bacteriorhodopsin and the D96N mutant over a wide pH range

We developed a new assay system for the measurement of capacitive electric currents generated by ion pumps using the thin polymer film 'Lumirror' (Toray Co., Japan). This system enables us to examine the electrogenicity of ion pumps over a wide range of experimental conditions with high reproducibility due to the mechanical and chemical stability, the high electric resistance and the high electric capacitance of the thin polymer film. Using this method, we examined the photoelectric response of wild type bacteriorhodopsin and its D96N mutant over a wide pH range (2.8-10.0). The results were explained in terms of the affinities of the proton binding sites for translocated protons. A possibility that the direction of the proton transfer from the Schiff base was influenced by the protonation/deprotonation state of the surrounding proton binding sites was suggested. We also found that this film can be used as a substrate for atomic force microscopy (AFM) samples and hence the active purple membrane was observed with AFM.

Significance of the glutamic acid residues Glu334, Glu959, and Glu960 of the alpha subunits of Torpedo Na+, K+ pumps for transport activity and ouabain binding

Glutamic acid residues in transmembrane segments of the alpha subunit of the Na+,K+-ATPase have been discussed as possible candidates for the binding sites of the transported cations. Here we report on effects of mutations of Glu334, Glu959, and Glu960 to alanine in ouabain-sensitive (OS) as well as ouabain-resistant (OR) ATPases of Torpedo electroplax expressed in Xenopus oocytes. All mutants are incorporated to about the same extend as the wild-type ATPases into the plasma membrane. None of the mutations produces complete inhibition of transport activity as judged from measurements of 86Rb+ uptake, membrane current, and ATPase activity. After conversion of OS to OR by mutation of the bordering residues of the first extracellular loop Gln118 to Arg and Asp129 to Asn, the Km value for inhibition by ouabain increases to 59 microM. Substitution of Glu334 to Ala in the OR pump variant restores ouabain sensitivity with a Km value of 0.12 microM, which is similar to that of the endogenous Xenopus pump. After substitution of Glu960 by Ala in the OR pump, ouabain sensitivity is partially restored. The Km values for pump stimulation by external K+ appear to be reduced in the OR compared to the OS pump. Mutation of Glu959 and Glu960 to Ala has no pronounced effects on the potential-dependent Km values at external pH 7.8; only in the Glu959-mutated OR pump, the apparent Km at 0 mV is raised. We conclude that none of the mutated glutamic acid residues is essential for cation coordination, but that GIu334, and in part also Glu960, seems to be involved in preserving the ouabain-resistant conformation of the enzyme.

Activation of the Na+/K+ pump current by intra- and extracellular Li ions in single guinea-pig cardiac cells

Li+ is the only ion that can replace the physiological intra- and extracellular activator cations of the Na+/K+ pump. In order to study this singular property of Li+ in some detail, the activation of the Na+/K+ pump current (Ip) by intra- and extracellular Li+ (Li+; Li[o]+) was measured in isolated guinea-pig ventricular myocytes by means of whole cell recording at 34 degrees C and a holding potential of -20 mV. Ip was identified as current blocked by dihydro-ouabain. Half-maximal Ip activation occurred at 23 mM Li(o)+ (K0.5 value) in cells containing Na+ (50 or 100 mM) and at 73 mM Li(o)+ in myocytes containing Li+ (100 mM). The K0.5 value of Ip activation by Li(o)+ increased with depolarisation, suggesting the transfer of 0.2 of an elementary charge across the electric field of the sacrolemma during Li(o)+-binding. An intracellular Li+ concentration of 36 mM caused half-maximal Ip activation in cells superfused with Na+- and Li+-free media containing 1 mM K+. In Na+-free solutions. the Ip-V curve displayed a positive slope at negative membrane potentials. A negative slope at positive potentials was observed in Li+-containing media. It is concluded that Li+ is less efficacious and potent than the physiological pump activator cations. The shape of the Ip-V curves in Na+-free solutions supports the view that the cardiac Na+/K+ pump contains a channel-like structure and suggests that there are voltage-sensitive steps in the pump cycle, apart from the binding of external cations.

Electrogenic K+ transport by the Na(+)-K+ pump in rat cardiac ventricular myocytes

1. The involvement of electrogenic reaction steps in K+ transport by the Na+, K(+)-ATPase was determined in rat cardiac ventricular myocytes using whole-cell patch clamp techniques. 2. Under K(+)-K+ exchange conditions and in the presence of extracellular K+ or Tl+ at concentrations that stimulated submaximal levels of steady-state Na+,K(+)-ATPase activity, ouabain-sensitive transient currents were observed during ('on') and after ('off') step changes in membrane potential (Vm). 3. The quantity of charge moved during the transient currents depended, in a saturable manner, on the magnitude of the voltage step. Maximal ouabain-sensitive 'on' and 'off' charges were calculated to be 9.6 +/- 0.9 and 9.1 +/- 0.4 fC pF-1 (n = 4), respectively, with an effective valeney of 0.48 +/- 0.07 (n = 7). 4. Kinetics of the transient currents were independent of Vm and Tl+o at positive potentials, but became more rapid at increasingly negative Vm values in an ion concentration-dependent fashion. 5. Those data demonstrate that electrogenic steps participate in K+ transport by the Na+,K(+)-ATPase and that the electrogenic step is extracellular ion binding. 6. The temperature- and Vm-dependent properties of transient charge movements were compared under K(+) -K+ and Na(+) -Na+ exchange conditions. The data suggest that extracellular K+ and Na+ binding occur at different sites in the enzyme or to different enzyme conformations. The sum of the effective valencies, 1.14 +/- 0.12, demonstrates that the electrogenicity of extra-cellular ion binding can explain the Vm dependence of ion transport by the Na+,K(+)-ATPase.

Change of Na+ pump current reversal potential in sheep cardiac Purkinje cells with varying free energy of ATP hydrolysis

1. The Na(+)-K+ pump current, Ip, of cardioballs from isolated sheep cardiac Purkinje cells was measured at 30-34 degrees C by means of whole-cell recording. 2. Under physiological conditions Ip is an outward current. Experimental conditions which cause a less negative free energy of intracellular ATP hydrolysis (delta GATP) and steeper sarcolemmal gradients for the pumped Na+ and Cs+ ions evoked an Ip in the inward direction over a wide range of membrane potentials. The reversal of the Ip direction was reversible. 3. The inwardly directed Ip increased with increasingly negative membrane potentials and amounted to -0.13 +/- 0.03 microA cm-2 (mean +/- S.E.M.; n = 6) at -95 mV. 4. The reversal potential (Erev) of Ip was studied as a function of delta GATP at constant sarcolemmal gradients of the pumped cations. 5. In order to vary delta GATP the cell interior was dialysed with patch pipette solutions containing 10 mM ATP and different concentrations of ADP and inorganic phosphate. The media were composed to produce delta GATP levels of about -58, -49 and -39 kJ mol-1. 6. A less negative delta GATP shifted Erev to more positive membrane potentials. From measurements of Ip as a function of membrane potential Erev was estimated to be -195, -115 and -60 mV at delta GATP levels of approximately -58, -49 and -39 kJ mol-1, respectively. The calculated Erev amounted to -224 mV at delta GATP approximately -58 kJ mol-1, -126 mV at delta GATP approximately 49 kJ mol-1 and -24 mV at delta GATP approximately -39 kJ mol-1. 7. Possible reasons for the discrepancy between estimated and calculated Erev values are discussed. 8. Shifting delta GATP to less negative values not only altered Erev but also diminished Ip at each membrane potential tested. The maximal Ip (Ip,max), which can be activated by external Cs+ (Cs+o), decreased under these conditions, whereas [Cs+]o causing half-maximal Ip activation remained unchanged. Similarly, the voltage dependence of Ip activation by Cs+o was unaffected. 9. It is concluded that Erev of Ip varies with delta GATP at constant sarcolemmal gradients of the pumped cations. This agrees with thermodynamic considerations.

Effect of Napip on K0 activation of the Na-K pump in adult rat cardiac myocytes

To determine if environmental factors influence the external K (K0) dependence of Na-K pump current (Ip), we systematically varied internal (pipette) Na (Napip) and Na-K pump activity while measuring the K0 dependence in adult rat cardiac myocytes. For each Napip, reactivation of Ip by K0 was dose dependent. The maximal Ip (Ipmax) and apparent affinity for K0 binding to the Na-K pump (K0.5) increased as Napip increased. The results of making an equimolar substitution of tetramethylammonium for K and Cs, and partial Ip inhibition with ouabain, also showed that Ipmax and K0.5 increased as Napip increased. We simulated pump activity as a function of intracellular Na (Nai) and K0 using a cyclic model of the Na-K pump and found that the model predicts K0.5 for K0 binding increases as Na increases, even when the conditions are adjusted by removing pipette K and partial pump inhibition with ouabain.

Electrogenic Na+/K(+)-transport in human endothelial cells

Na+/K+ pump currents were measured in endothelial cells from human umbilical cord vein using the whole-cell or nystatin-perforated-patch-clamp technique combined with intracellular calcium concentration ([Ca2+]i) measurements with Fura-2/AM. Loading endothelial cells through the patch pipette with 40 mmol/l [Na+] did not induce significant changes of [Ca2+]i. Superfusing the cells with K(+)-free solutions also did not significantly affect [Ca2+]i. Reapplication of K+ after superfusion of the cells with K(+)-free solution induced an outward current at a holding potential of 0 mV. This current was nearly completely blocked by 100 mumol/l dihydroouabain (DHO) and was therefore identified as a Na+/K+ pump current. During block and reactivation of the Na+/K+ pump no changes in [Ca2+]i could be observed. Pump currents were blocked concentration dependently by DHO. The concentration for half-maximal inhibition was 21 mumol/l. This value is larger than that reported for other tissues and the block was practically irreversible. Insulin (10-1000 U/l) did not affect the pump currents. An increase of the intracellular Na+ concentration ([Na+]i) enhanced the amplitude of the pump current. Half-maximal activation of the pump current by [Na+]i occurred at about 60 mmol/l. The concentration for half-maximal activation by extracellular K+ was 2.4 +/- 1.2 mmol/l, and 0.4 +/- 0.1 and 8.7 +/- 0.7 mmol/l for Tl+ and NH4+ respectively. The voltage dependence of the DHO-sensitive current was obtained by applying linear voltage ramps. Its reversal potential was more negative than -150 mV. Pump currents measured with the conventional whole-cell technique were about four times smaller than pump currents recorded with the nystatin-perforated-patch method.(ABSTRACT TRUNCATED AT 250 WORDS)

Na+ pump current-voltage relationships of rabbit cardiac Purkinje cells in Na(+)-free solution

1. The Na+ pump current (Ip) of isolated, single rabbit cardiac Purkinje cells in Na(+)-free solution was measured at 32-34 degrees C by means of whole-cell recording. 2. The Ip amplitude was studied as a function of clamp potential (Vc) and external concentration of various monovalent cations known to activate the Na(+)-K+ pump. 3. Under conditions which strongly activated Ip the Ip-Vc curve of the cells displayed a positive slope at membrane potentials negative to -20 mV and little variation at more positive potentials. 4. The Ip-Vc relationship showed an extended region of negative slope at positive and negative potentials in solutions containing low concentrations of activator cations which caused little Ip activation. A positive slope of the Ip-Vc curve was occasionally observed at clamp potentials negative to -60 mV under these conditions. 5. The shape of the Ip-Vc relation was independent of the cation species used as external Ip activator. 6. At zero membrane potential half-maximum Ip activation (K0.5(Vc = 0 mV) occurred at 0.05 mM Tl+, 0.08 mM K+, 0.4 mM NH4+ and 1.5 mM Cs+. The Hill coefficient derived amounted to 0.9 for Tl+, 1.2 for K+, 1.04 for NH4+ and 1.5 for Cs+. 7. The concentrations of external activator cations required for half-maximum Ip activation increased with depolarization. The voltage dependence of the K0.5 values could be described by a single exponential function for clamp potentials positive to -40 mV. 8. The steepness of the function is determined by a factor alpha, indicating the apparent fraction of an elementary charge which moves in the electrical field across the sarcolemma when external monovalent cations bind to the Na(+)-K+ pump. 9. The alpha values were calculated to be 0.32 for Tl+, 0.24 for K+, 0.29 for NH4+ and 0.18 for Cs+. Possible interpretations of the alpha values are considered. 10. It is suggested that binding of external monovalent activator cations to the Na(+)-K+ pump (or a process related to the binding) is voltage dependent. This potential-dependent process determines mainly the shape of the Ip-Vc curve in cardiac Purkinje cells superfused with Na(+)-free media containing low concentrations (< K0.5(Vc = 0 mV)) of K+ or its congeners.